Wig Dye Kits
These can be found online or at wig stores, and can also be used to change the color of wigs. For a rough pricing estimate, Karie Blair sells her 4oz kits at $16

Remember: Synthetic wigs can only go a darker color, not lighter. This is because the color is in the plastic and can’t be removed. For example, wigs that are a dark blue cannot become a light blue because you can only add more color on top. For lighter wig colors start with a white base, for darker colors blonde can help with giving you a slightly darker result.

Please note that wig dyeing can be very messy, especially the sharpie and spraybottle method. It is best to do this outside or have a drop cloth to avoid getting the dye everywhere.